Automatic, powershifting transmissions for vehicles, called automatic transmissions for short, usually have a hydrodynamic torque converter as the starting element. Predominantly, these transmissions are designed as range-change transmissions, which comprise a plurality of planetary gearsets for obtaining a number of gears or gear steps which are usually engaged by means of hydraulic shifting elements, for example disk clutches or disk brakes.
To operate correctly a transmission requires an effective oil supply system for lubrication and cooling, and particular filling quantities and oil pressures for shifting the hydraulic components. For that purpose, as a rule a hydraulic pump is provided that is coupled to a transmission input shaft and driven by an internal combustion engine, and which supplies the requisite oil pressure by way of a regulated, valve-controlled hydraulic system.
To reduce the emission of pollutants, fuel consumption and noise levels, it is moreover desirable, particularly in urban traffic, to provide for so-termed start-stop operation in which, depending on the situation and possibilities, the combustion engine should be switched off when the vehicle is at rest, for example at road junctions controlled by traffic lights. However, in vehicles with converter automatic transmissions this is not straightforward.
Since the hydraulic pump is powered by the combustion engine, pressurized oil is only available while the engine is running. In contrast, when the combustion engine is switched off the hydraulic system loses pressure, whereupon the transmission control system usually disengages the current gear and shifts the transmission to a neutral position. Before any torque can be transmitted again a certain pressure must first build up again in the hydraulic system before the vehicle can start off. This results in a relatively long activation time before the gear is engaged after the engine has been started, and this makes it virtually impossible to operate a start-stop function with a high frequency of starting processes and the need, when the engine has been started, for example when the traffic lights change, to move off as quickly as possible since in practice frequent and long delays occur.
To enable start-stop operation, an additional pump powered by an electric motor could be used, which maintains the oil pressure when the internal combustion engine is switched off. However, besides additional costs this would take up more installation space and increase both weight and energy consumption, and would therefore have a counterproductive effect on the energy balance of the start-stop function, so that where possible it should be avoided.
DE 10 2007 003 924 A1 discloses a hydraulic system of an automatic transmission with a hydraulic pump that can be driven by an internal combustion engine for the pressurizing and cooling of hydraulic shifting elements of the automatic transmission. In order to delay a pressure drop in a main pressure circuit when the combustion engine is switched off, according to DE 10 2007 003 924 A1 it is proposed to connect the main pressure circuit by way of a connection line having a one-way valve, to a retarder reservoir. A disadvantage of this, however, is that an additional connection line and a one-way valve arranged in the connection line are needed in the hydraulic system.